Unit 1 Physics: Motion and Energy- Forms and Changes

Physics – Motion and Energy

Here’s what you will know when you are done with this unit!

I. Review motion in one dimension by analyzing how distance, displacement, speed, velocity acceleration and time interrelate with each other. II. Determine the difference between a vector and scalar quantityIII. Use scientific principles to solve a problem by using a technological design. IV. Understand and use changes in variables that affect a system to solve problems.V. Understand the forms of energy found in our natural world. VI. Understand how energy can change forms but never be lost

VII. Explore how kinetic and potential energy can be used to solve problems

 

 

Physics is the study of matter and energy and their

relationships

What is Physics?

Everybody uses physics everyday on a simple level. When ever we try to solve a problem in the physical world we use physics to solve the problem.

This could be from throwing down sand and salt on ice to knowing which way to push to run forward.

Most of the things in Physics we have intuitively learned when we were just a few years old. This year we will try to explain why they work.

Who Uses Physics?

In physics, if we are to ask questions we should try to consider everything

To describe motion accurately and completely, a coordinate system is necessary.

Coordinate system tells you the location of a zero point of the variables you are studying and the direction in which the values of the variables increase.

An example of this is a graph or a map

Coordinate systems

A. Motion Diagram (1D) Indicate the position of something at equal

intervals of time Ex.

1s 2s 3s 4s 5s

How Things Move

Vectors – quantities that have both size (also called magnitude) and a direction

Ex – velocity (meters per second north)

Scalars – quantities that have only magnitude

Ex – speed- (meters per second)

Vectors and Scalars

Movement in relation to a frame of reference.

Example – people in a car look like they are moving to somebody standing on the road, but that person doesn’t seem to be moving when looking at somebody right next to them

Relative Motion and Frame of reference

Distance – length of a path between two points

SI unit of distance is meters Scalar quantity – gives only magnitude Shows total amount of movement no matter

without concern for direction. Example - It’s five blocks away

Distance

Displacement – distance and direction Shows movement from where the object

started from Units are also meters Vector quantity – quantity that has magnitude

AND direction. Example – 5 blocks North

Displacement

When two displacements have the same directions they can be added.

When two displacements have different directions they can be subtracted.

When two or more displacements have different directions, they may be combined by graphing.

Resultant vector – the vector sum of two or more vectors.

Adding displacements

Speed, Velocity, and Acceleration

Speed is the distance traveled in a certain amount of time. It is a scalar quantity It does not tell you direction.

Scalar quantity s= Speed d= distance t = time

s = d/t

What is speed?

You Try!

If Johnny is riding 3 kilometers per hour on his skateboard, what is his speed in kilometers/hour?

3 kilometers/ hourIf Tanya can ride her bmx bike 10 miles in 2

hours, what is her speed in miles per hour? 5 miles / hour

If a space ship travels towards the sun 150,000 km per day, what is the speed in km/h?

6250 km/ h

Velocity is speed AND direction! v = velocity d = displacement ( this deals with direction also. t = time

v = d / t (don’t forget that you must include direction.

Velocity is a vector quantity

What is velocity?

v = ∆d / ∆ t = d1 – d0/ t1 – t0

= change

You try! Find the average velocity for a biker who biked 5 miles the first 10 minutes, 3 miles the second 10 minutes, and 1 mile the third 10 minutes. The biker was traveling east.

Average Velocity

Velocity within the time interval v = instantaneous velocity v = d / t within a certain localized time

interval.

Instantaneous velocity

How much something is speeding up or slowing down, changes in direction, or changes in both speed and direction.

Acceleration is a vector a = acceleration ∆v = change in velocity t = time interval a = v / t

Acceleration

Average acceleration is the average of the acceleration over an entire period of time

average acceleration equals the change in velocity divided by the change in time.

a = v/ t = vf – vi/ tf – ti

Average Acceleration

Find the acceleration of a foam ball dropping at 3 m/s in 1 second?

3 m/s2

Calculate the acceleration of a car (in km/h∙s) that can go from rest to 100km/h in 10 s.

(10 km/h·s)

You Try

 

Instantaneous Acceleration

Changing direction also is acceleration Since the object changes direction, the

vector of the object changes causing it to have a change in velocity in that plane which causes a change in acceleration.

Changing direction

Constant acceleration – there is a steady change in acceleration meaning that the velocity of an object changes by the same amount each second.

Acceleration

Free fall is the acceleration of an object towards the earth solely because of gravity

Objects that are near Earth’s surface accelerate at a rate of 9.8 m/s2 if we are in a vacuum.

Free Fall

Much more spectacular differences in the strength of gravity can be observed away from the Earth's surface:

Location g (m/s2) asteroid Vesta (surface)……….0.3 Earth's moon (surface)………..1.6 Mars (surface)………………...3.7 Earth (surface)………………..9.8 Jupiter (cloud-tops)……………26 Sun (visible surface)………….270 typical neutron star (surface)…1012 black hole (center)infinite according to some theories,

on the order of 1052 according to others

Comparing Gravity

There are three simple equations that can be used to help understand one relationship with another

Equation #1 Final Velocity = Initial velocity + acceleration x change in time or the formula

vf = vi + aΔt

Linking them all together

 

Equation #2

The square of the final velocity equals the sum of the square of the initial velocity and twice the product of the acceleration and the displacement since the initial time.

Vf2 = Vi

2 + 2a(df-di)

Equation #3

Work and Energy

Nature of Energy

Energy is all around you! You can hear energy as sound. You can see energy as light. And you can feel it as wind.

What is work

Work is equal to the constant force exerted on an object in the direction of motion, times the object’s displacement

Measured in joules

W = Fd W = Δ KE

What is energy?

Energy is the ability of an object to produce a change in itself or the world around it.

Also defined as the change of work in a system

Power

 

Kinetic Energy

The energy resulting from motion

Equal to ½ times the mass of the object multiplied by the speed of the object squared

KE = ½ mv2

Measured in joules Also useful is W = Δ

KE

Gravitational Potential Energy

Gravitational potential energy of an object is equal to the product of its mass, the acceleration due to gravity and the distance from the reference level.

PE = mgh Also measured in

joules

Elastic Potential Energy

The potential energy that may be stored in an object such as a rubber band, as a result of its change in shape

Examples: a pulled back rubber band, rubber balls, trampolines

Thermal Energy

A measure of the internal motion of an object’s particles

A change in an object’s thermal energy is measured with a thermometer

Kinetic-Potential Energy Conversions

As a basketball player throws the ball into the air, various energy conversions take place.

Let’s Practice  

Car’s mass = 1550kg

Potential Energy

Gravitational Potential Energy is energy due to an objects height above the ground

PE = mghm = massg = acceleration due to

gravityh = height

http://gaaf.com/pictures/200406_utah/image009.htm

Let’s Practice

A rock has a mass of 8.40 x 104 kg. The center of mass is 29.0 m above the ground. How much energy does it have?

PE = mghIdentify the variablesm= 8.40 x 104 kgg= 9.81 m/s2

h= 29.0 mPE = (8.4X104)(9.81)(29) = 2.39 x 107 J

http://gaaf.com/pictures/200406_utah/image009.htm

We call the sum of PE and KE mechanical energy.ME = KE + PE

Mechanical energy is important because it is conserved (as long as there are no non conservative forces, like friction)

Therefore, if one goes down, the other goes up by the same amount.

What’s so important about PE and KE?

Conservation of Energy

The law of conservation of energy states that in a closed, isolated system, energy can neither be created nor destroyed; rather energy is conserved

Energy just changes form from one to another while the total stays the same.

Conservation of Mechanical Energy

When mechanical energy is conserved, the sum of the kinetic energy and potential energy before an event is equal to the sum of the kinetic and potential energy after the event. KEb +PEb = KEa + PEa

Energy can be changed from one form to another. Changes in the form of energy are called energy conversions.

Energy Conversion

All forms of energy can be converted into other forms. The sun’s energy through solar cells can be

converted directly into electricity. Green plants convert the sun’s energy

(electromagnetic) into starches and sugars (chemical energy).

Energy conversions

Conceptual understanding

A penny is dropped off the Eiffel tower (ignore air resistance). As it falls, what happens to it’s potential energy? What happens to it’s kinetic energy?

As it falls, its velocity goes up, so its kinetic energy goes up. It also looses height so its potential energy goes down.

However, mechanical energy stays the same ME = KE + PE

Kinetic vs. Potential Energy

At the point of maximum potential energy, the car has minimum kinetic energy.

At the point of maximum kinetic energy, the car has minimum potential energy.

The total mechanical energy stays the same. (Energy cannot be created nor destroyed.

In 1905, Albert Einstein said that mass and energy can be converted into each other.

He showed that if matter is destroyed, energy is created, and if energy is destroyed mass is created. Energy equals the speed of light multiplied by the mass of the object in kilograms.

E = mC2

Einstein’s Take on Law of Conservation of Energy